An aircraft is driven by a plurality of jet engines, each of which is accommodated in a nacelle.
A nacelle generally has a tubular structure, comprising an air inlet which is upstream from the jet engine, a mid-section which is designed to surround a fan of the jet engine, and a downstream section which optionally incorporates thrust reversal means, is designed to surround the combustion chamber of the jet engine, and generally ends in an exhaust nozzle, the outlet of which is situated downstream from the jet engine.
Modern nacelles are designed to accommodate a dual flow jet engine, which can firstly generate a hot air flow (also known as the primary flow) which is obtained from the combustion chamber of the jet engine, and secondly a cold air flow (secondary flow) which is obtained from the fan, and circulates on the exterior of the jet engine by means of an annular passage, also known as the gap, formed between an inner structure which defines a fairing of the jet engine, and an inner wall of the nacelle. The two air flows are discharged from the jet engine from the rear of the nacelle.
The role of a thrust reverser is, when an aircraft is landing, to improve the braking capacity of the latter, by redirecting forwards at least part of the thrust which is generated by the jet engine. In this phase, the reverser obstructs the stream of the cold flow, and directs it towards the front of the nacelle, thus generating a counter-thrust which is added to the braking of the wheels of the aircraft.
The means which are implemented in order to obtain this reorientation of the cold flow vary according to the type of reverser. However, in all cases, the structure of a reverser comprises movable cowls which can be displaced between firstly a deployed position, in which they open up in the nacelle a passage which is destined for the deflected flow, and secondly a retracted position, in which they close this passage. These cowls can fulfil a function of deflection, or simply of activation of other deflection means.
In the case of a reverser with grids, which is also known as a cascade reverser, the reorientation of the flow of air is carried out by deflection grids, and the cowl has only a simple sliding function which uncovers or re-covers these grids. Complementary locking doors, which are also known as shutters, and which are activated by the sliding of the cowl, generally permit closing of the gap downstream from the grids, such as to optimise the reorientation of the cold flow.
During maintenance operations on a jet engine and the nacelle which surrounds it, it is important to ensure that the movable cowls cannot open unexpectedly, which would constitute a danger for the operators.
Similarly, when the internal safety mechanisms of the reverser are damaged, which mechanisms are known as the PLS (Primary Lock System) and TLS (Tertiary Lock System), it may be preferable to inhibit the movable cowls mechanically rather than risk deployment in flight, which would be catastrophic.
The inhibition of the movable reverser cowls during maintenance operations can be obtained by means of electrical and/or mechanical inhibition systems. Inhibition of the reverser in flight is preferably carried out mechanically.
Mechanical inhibition of this type is generally carried out by screwing movable cowls onto a fixed structure of the nacelle, and/or by putting locking studs into place downstream from the movable cowls according to a substantially radial direction, the said studs then acting as stop means which prevent any longitudinal translation movement of the movable cowl.
A system of this type has numerous disadvantages.
Firstly, the studs or screws must be stored on the aircraft, generally inside a receptacle which is provided in the nacelle.
Then, putting the studs into place or screwing on of the cowls is a lengthy and tedious operation which requires suitable tooling.
It should also be noted that the new generation of large carriers is equipped with particularly powerful jet engines, which are surrounded by a nacelle with large dimensions. Their movable cowls are therefore all the heavier, and their mechanical inhibition requires putting into place of particularly strong retention means. This results in the need for additional locking studs or screws, which is not desirable.
It will also be noted that the movable cowls of thrust reversers are generally in the form of two semi-cylindrical cowls which slide on a minimal fixed structure comprising one or a plurality of upper beams which are situated at approximately twelve o'clock in the region of the strut, and one or a plurality of lower beams which are situated at approximately six o'clock, with the said sliding beams supporting rails to guide the movable cowls. For reasons of synchronisation of the movable cowls, and of safety, the two semi-cylindrical movable cowls can be connected mechanically. It is therefore necessary to provide studs or screws which, in the event of deficiency of the other studs or screws, can withstand all of the force which is generated by all of the jacks dedicated to the deployment of the movable cowls.
Thus, whereas an obvious solution could consist of reducing the number of studs or screws to the point where optionally only one of them would then be needed, the size of this stud would have to be such as to be able to bear the weight of two movable cowls. In fact, another problem is derived from the fact that the screws or studs which are used conventionally generally have a cylindrical or conical shape, and consequently have to withstand substantial shearing forces when they are subjected to stress by the movable cowl. A second problem therefore consists of improving the distribution of the forces which are exerted on the stud, in order to be able to optimise the dimensions of the latter.